> > On Tue, Jul 04, 2023 at 04:26:15PM +0800, Zqiang wrote: > > When invoke synchronize_srcu(), in the srcu_might_be_idle(), the current > > CPU's sdp->lock will be acquired to check whether there are pending > > callbacks in the sdp->srcu_cblist, if there are no pending callbacks, > > probabilistically probe global state to decide whether to convert to > > synchronize_srcu_expedited() call. however, for the rcupdate.rcu_normal=1 > > kernels and after the rcu_set_runtime_mode() is called, invoke the > > rcu_gp_is_normal() is always return true, this mean that invoke the > > synchronize_srcu_expedited() always fall back to synchronize_srcu(), > > so there is no need to acquire sdp->lock to check sdp->srcu_cblist and > > probe global state in srcu_might_be_idle(). > > > > This commit therefore make srcu_might_be_idle() return immediately if the > > rcu_gp_is_normal() return true. > > > > Signed-off-by: Zqiang <qiang.zhang1211@xxxxxxxxx> > > --- > > kernel/rcu/srcutree.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > > index 20d7a238d675..aea49cb60a45 100644 > > --- a/kernel/rcu/srcutree.c > > +++ b/kernel/rcu/srcutree.c > > @@ -1172,6 +1172,8 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp) > > unsigned long tlast; > > > > check_init_srcu_struct(ssp); > > + if (rcu_gp_is_normal()) > > + return false; > > Again, thank you for looking into SRCU! > > I am not at all enthusiastic about this one. With this change, the name > srcu_might_be_idle() is no longer accurate. Yes, the name could change, > but any name would be longer and more confusing. > > So unless there is a measureable benefit to this one on a production > workload, I cannot justify taking it. > > Is there a measureable benefit? > Hi, Paul I only find that for Preempt-RT kernel, the rcu_normal_after_boot is set by default: static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT); This affects only rcu but also srcu, this make the synchronize_srcu() and synchronize_srcu_expedited() always fall back to __synchronize_srcu(ssp, true), this means that call the srcu_might_be_idle() is meaningless. Thanks Zqiang > > Thanx, Paul > > > /* If the local srcu_data structure has callbacks, not idle. */ > > sdp = raw_cpu_ptr(ssp->sda); > > spin_lock_irqsave_rcu_node(sdp, flags); > > -- > > 2.17.1 > >
